In the co-pending, commonly assigned, applications Ser. No. 657,888, filed Oct. 5, 1984, now Pat. No. 4,715,261 entitled "Cartridge Containing Plasma Source for Accelerating a Projectile" and Ser. No. 809,071, filed Dec. 11, 1985, now abandoned, entitled "Plasma Propulsion Apparatus and Method" there are disclosed an apparatus for and method of accelerating a projectile with a high pressure plasma produced in response to a high voltage discharge. The projectile is located in a gun barrel downstream of a high pressure source of ionized gas including a capillary passage, i.e., a passage having a length to diameter ratio of at least 10:1. The passage includes ionizable material, preferably low atomic weight elements in molecules forming the passage wall. The low atomic weight elements (e.g. hydrogen and carbon) are ablated from the wall in response to a high voltage discharge established between spaced first and second electrodes, respectively located at open and closed ends of the passage. The passage dimensions and ionized materials cause the discharge to have a relatively high resistance, such as 0.1 ohm. A gas having a high pressure, such as in excess of 100 bars, is established in the capillary passage and escapes through the open end of the passage to accelerate a projectile in a gun barrel, usually made of steel, located immediately downstream of the open end.
To maximize the projectile velocity, an electric ionizing pulse supplied to the electrodes is shaped so the pressure behind the projectile in the barrel is maintained substantially constant despite the increasing volume in the barrel behind the moving projectile. The electric pulse is shaped so the power applied to the discharge increases substantially linearly as a function of time while the projectile is being accelerated through the barrel. The pulse is terminated prior to the projectile reaching the muzzle end of the barrel, approximately at the time that the projectile has traversed approximately one-half of the barrel length.
A confined mass of evaporable, ionizable material is located between the open end of the capillary tube and the back end of the projectile. This mass of material is evaporated and then ionized by plasma in the discharge to add additional propulsive force to the projectile and to cool the plasma discharge escaping from the open end of the capillary passage. Preferably, the evaporable material between the open end of the capillary and the back end of the projectile includes low atomic weight elements, such as hydrogen and carbon.
While the prior art structures have functioned satisfactorily for many purposes, they have generally been limited to accelerating projectiles to about 5 kilometers per second because high atomic weight elements, e.g., iron, of the gun barrel are melted and evaporated by the plasma to reduce the sound speed of the projectile accelerating gas. To avoid barrel wall melting, it is necessary to maintain the temperature of gas flowing into the gun barrel to below about 3500.degree. K. This limitation of 3500.degree. K. translates into a projectile velocity limitation of about 5 kilometers per second for hydrogen-rich flows.
It is, accordingly, an object of the present invention to provide a new and improved apparatus for and method of accelerating a projectile through the use of electric discharge plasmas.
Another object of the invention is to provide a new and improved apparatus for and method of enabling high pressure gases to be built up behind a projectile in response to a plasma discharge that initiates the plasma.
Another object of the invention is to provide a new and improved apparatus for and method of accelerating projectiles to speeds in excess of 10 kilometers per second.
An additional object of the present invention is to provide a new and improved apparatus for and method of accelerating projectiles wherein the projectile velocity is not limited by melt characteristics of an elongated barrel downstream of a high pressure, high temperature plasma source.
In the copending, commonly assigned, application Ser. No. 06/929,365, filed Nov. 12, 1986, entitled "Apparatus for and Method of Accelerating a Projectile Through a Capillary Passage and Projectile Therefor", the prior art is modified so the projectile initially resides in the capillary passage and a discharge current is established between spaced regions along the passage through the projectile. In particular, an apparatus for accelerating a projectile comprises a structure having a dielectric wall forming a capillary passage. Low atomic weight ionizable elements (e.g. hydrogen and carbon) in the passage, preferably in the form of ablatable material on the tube wall, are ionized to form a plasma in response to a discharge formed between first and second electrodes at spaced regions along the passage length. One end of the tube is closed and the other end of the tube is open so that a projectile in the passage can be projected through it. The discharge established along the length of the passage between the electrodes includes electrons that are conducted through the projectile. The discharge ionizes and ablates material from the wall in front and in back of the projectile to form a high pressure gas in a confined region behind the projectile. The pressure in the region behind the projectile is much higher than the pressure in the passage in front of the projectile to accelerate the projectile at a high speed along the passage and through the passage open end; the projectile escape velocity from the passage open end, i.e., the projectile muzzle velocity, can exceed 10 kilometers per second. Only low atomic weight elements, e.g. hydrogen and carbon, are in the high pressure plasma accelerating the projectile so that high atomic weight elements are not injected into the plasma. Thereby, the sound speed of the accelerating gas is not reduced by foreign materials and the stated very high projectile velocity is attained.
A confined mass of low atomic weight evaporable ionizable substance is in the tube passage behind the projectile. The substance is evaporated to establish a high pressure behind the projectile to initially accelerate the projectile toward the open end. The projectile is thereafter accelerated by the high pressure gas in the confined region behind the projectile resulting from the plasma formed by the ablated material. The substance is ionized by a discharge, which may or may not be the same discharge as the discharge which establishes the plasma discharge between the electrodes.
The ionized substance and the ablated, ionized material behind the projectile form a high resistance, high pressure plasma gas behind the projectile. Ionized material in the passage in front of the projectile is a low resistance plasma having relatively low pressure. Thereby, the gases behind the projectile are subjected to greater ohmic heating than gases in front of the projectile to assist in providing a greater pressure behind the projectile than in front of the projectile. The ablated gaseous material in the passage in front of the projectile is very hot and therefore flows rapidly out of the muzzle without being pushed by the projectile, so that it does not impede the projectile movement through the passage. The confined substance is such that the discharge in the passage behind the projectile is relatively cool to assist in maintaining a high electrical resistivity and high pressure in the gases behind the projectile.
Preferably, the substance is in fluidizable form, so it has a large surface area when subjected to the plasma. This enables the plasma to have an initial relatively low temperature and therefore relatively high electrical resistance that promotes ohmic heating of the plasma. There is a controlled release of plasma from the fluidizable substance. If the substance is a fluidizable particulate, the plasma release rate is a function of the size of the particles in the substance and the amplitude of the discharge current in the substance. The discharge current amplitude is shaped in a predetermined manner to control the plasma release for the size range of the particles. Small grains, e.g., particles having a diameter of about 20 microns, evaporate considerably faster and generate plasma at a faster rate than large grains, e.g., of 100 micron diameter.